Planar surface shapes, which can be readily matched by optical wavefronts, are especially suitable for measurement by interferometers. Among these are reflective optical flats and prisms, as well as non-optical elements having flat surfaces. However, the near symmetry of transmissive plane-parallel plates, such as glass plates, presents special challenges and opportunities for interferometric measurement.
The parallel surfaces of such plates reflect light in parallel directions, so the light reflected by the two surfaces is difficult to distinguish. Fizeau interferometers exploit this property by comparing reflections from both surfaces to measure small wedge angles between the surfaces. Twyman-Green interferometers pass light through both plane-parallel surfaces to measure homogeneity of the material supporting them. However, surface topologies are generally measured one surface at a time to avoid confusion between the surfaces.
For example, Newton interferometers, which use a broad band light source, position a reference flat against one surface of a plane-parallel plate at a time to separately measure the surface topologies of the parallel surfaces. Other white light interferometers eliminate unwanted interference fringe patterns from parallel surfaces of transmissive plates by limiting the production of interference fringe patterns to reference and test surfaces located at equal optical path lengths along reference and test arms. Well-matched optics are required between the reference and test arms, which can be prohibitively expensive for measuring large test plates.
Interferometers with temporally coherent light sources generally rely on light-scattering coatings to distinguish the parallel surfaces. The coatings are applied to one of the parallel surfaces to scatter potentially interfering light from the coated surface. However, the coatings take time to apply, can warp or contaminate the plates, and can be difficult to remove.
U.S. Pat. No. 5,448,477 to de Groot discloses a Fizeau interferometer operating with software that mathematically separates the interference contributions of plane-parallel surfaces. However, the measurement of either or both parallel surfaces of a test plate requires a sequence of phase-shift measurements and inversion of the test plate for measuring both surfaces in two opposite orientations.
Commonly assigned U.S. Pat. No. 4,325,637 to Moore uses limited spatial coherence in a grazing incidence interferometer to relatively increase the fringe visibility of an interference pattern generated between a reference surface and one of the plane-parallel surfaces of a test plate with respect to an interference pattern between the same reference surface and the other plane-parallel surface of the test plate. However, the technique sometimes does little to reduce the fringe visibility of yet another interference pattern between the two parallel surfaces of the plate.